There is a ball SAW sensor (spherical Surface Acoustic Wave device) that uses the change in the propagation speed depending on the ambient hydrogen concentration when a surface acoustic wave (SAW) propagates on the surface of a sphere made from a single crystal piezoelectric material such as liquid crystal, langasite, LiNbO3, LiTaO3, or the like. (See Japanese Unexamined Patent Application Publication 2003-115743 and Japanese Unexamined Patent Application Publication 2005-291955). When there is excitation of a surface acoustic wave on the surface of the sphere, the surface acoustic wave does not spread out as would a normal wave, but rather travels around a circular region, with a limited width, along the great circle of the sphere, around a specific crystallographic axis, many times, essentially without attenuation. The ball SAW sensor is an extremely sensitive hydrogen sensor because the change in the aforementioned propagation speed increases proportionately with the number of times that the surface acoustic wave has circled the sphere.
FIG. 6 illustrates schematically the structure of the surface acoustic wave device. A comb electrode 12 and a sensitive membrane 13 are formed on a spherical base member 11 made out of a piezoelectric material single crystal. The sensitive membrane 13 is made out of Pd, Ni, Pd—Ni alloy, or the like, that absorbs hydrogen. Because a sensitive membrane 13 that has absorbed hydrogen becomes rigid, causing the speed of propagation of the surface acoustic wave in the sensitive membrane 13 to become faster, this can be used as a hydrogen sensor. Here the comb electrode 12 and the sensitive membrane 13 must be formed in specific locations on the base member 11. Specifically, the comb electrode 12 and the sensitive membrane 13 are formed on the equator that has, as the axis thereof, the optical axis 14 that passes through the center of the sphere (termed simply the “equator,” below), as illustrated in FIG. 6. In the specification, the optical axis that passes through the center of the sphere shall be termed simply the “optical axis.” In particular, it is necessary to form the comb electrode 12 with high precision, because there is a sharp decline in the sensitivity of the ball SAW sensor when, in particular, the position at which the comb electrode 12 is inaccurate. Note that piezoelectric materials such as liquid crystal, langasite, LiNbO3, LiTa3, and the like are optically uniaxial crystals, and thus possess a single optical axis.
Here, the optical axis 14, for example, is detected in order to determine the position wherein the comb electrode 12 is to be formed. The comb electrode 12 is formed at a position that is rotated by 90° from the detected optical axis 14, or in other words, on the equator. The sensitivity of the device is reduced, and uniform quality cannot be maintained, if the position wherein the comb electrode 12 is formed is not precise. Because of this, there is the need to detect the optical axis 14 accurately, and the methods for detecting the optical axis described in Japanese Patent Application 2006-322993 and Japanese Patent Application 2007-253006 have been used by the authors. The optical axis can be detected easily and accurately through these methods.
As described above, it is necessary to specify the equator from the detected optical axis 14, and necessary to form the comb electrode 12 thereon. Specifically, after detecting the optical axis 14 of a base member 11 that has a diameter of 1 mm, the base member 11 is held and transferred to the next process, and in the next process, the comb electrode 12 is formed in a position that is 90° from the detected optical axis 14. In this case, the error in each process is cumulative, and thus there is the danger that the position at which the comb electrode 12 is formed may not be accurate.
The object of the present invention is to provide a method for detecting the equatorial plane, capable of detecting directly an equatorial plane that has the optical axis as the axis thereof, in a spherical optically uniaxial crystal.